Nanoparticle doping as a way to enhance holmium fiber lasers efficiency

Highly-doped holmium fibers have been prepared using modified chemical vapor deposition in combination with nanoparticle-doping method. Within a series of various Al₂O₃ and Ho³⁺ concentrations, relations between fibers composition and their fluorescence and laser parameters have been studied. Al/Ho molar ratio equal to at least 50 was found to be the key factor for fibers with outstanding parameters. Fibers with slope efficiency above 80%, laser threshold below 100 mW and fluorescence lifetime up to 1.6 ms have been prepared. Thanks to high Al₂O₃ concentrations, obtained through nanoparticle doping, we were able to achieve high-performance fibers in a wide range of Ho³⁺ concentrations. An output power of 19 W with 81% slope efficiency was reached using fiber with almost 4000 ppm of Ho³⁺ and 10 mol.% of Al₂O₃. This result is encouraging for highly efficient high-power cladding-pumped holmium fiber lasers, and studied relations between fibers composition and their laser parameters will be used in the designing of such laser sources.